In general, the inverter power source control circuit employed in a high frequency heating apparatus such as, for example, a microwave oven is so designed that an alternating current output from an alternating current power source is full-wave rectified to provide a direct current output on one hand and, in order to convert this direct current output to a desired alternating current output that is to be supplied to a magnetron employed in the microwave oven, an oscillation control signal based on a PWM control signal fed from a microcomputer used to control the microwave oven is applied through an inverter to a switching element such as an IGBT (Insulated Gate Bipolar Transistor) element See, for example, the Japanese Laid-open Patent Publication No. 2002-56966.
However, the magnetron employed in the microwave oven is generally known to have such a characteristic that no electric current flows therethrough unless an electric voltage of a value equal to or higher than a predetermined value is applied thereto. For this reason, the alternating current output from the alternating current power source represents such a distorted waveform as shown in FIG. 6B. Specifically, considering that the electric current abruptly starts flowing when the electric voltage attains a value equal to or higher than the predetermined value, but fails to flow when the voltage attains a value lower than the predetermined value, the distorted waveform of the alternating current output involves steep rise (ascent) and descent areas appearing on both sides of the waveform. Once this distorted waveform occurs in the alternating current output, the power factor of the electric power source circuit fails to increase. In addition, according to the state of the art, since no influence brought about on the power factor by fluctuation of the PWM control signal is virtually taken into consideration, a difficulty is keenly realized in increasing the efficiency of the electric power supply in a large range of load.
Also, when it comes to the microwave oven for household use, needs for the swift cooking capability and the safety factor are of primary concern and, therefore, the inverter power source control circuit employed in the microwave oven for household use is desired to have a high stability against and a high responsivity with change in output from the magnetron used therein.